Technical Field
This disclosure relates to precision metrology using a machine vision inspection system, and more particularly to automatically determining an approximate adjustment distance in order to bring the workpiece to be inspected into a desirable focus range in a machine vision inspection system.
Description of the Related Art
Precision non-contact metrology systems such as precision machine vision inspection systems (or “vision systems” for short) may be utilized to obtain precise dimensional measurements of objects and to inspect various other object characteristics, and may include a computer, a camera and optical system, and a precision stage that moves to allow workpiece traversal and inspection. One exemplary prior art system, characterized as a general-purpose “off-line” precision vision system, is the QUICK VISION® series of PC-based vision systems and QVPAK® software available from Mitutoyo America Corporation (MAC), located in Aurora, Ill. The features and operation of the QUICK VISION® series of vision systems and the QVPAK® software are generally described, for example, in the QVPAK 3D CNC Vision Measuring Machine User's Guide, published January 2003, and the QVPAK 3D CNC Vision Measuring Machine Operation Guide, published September 1996, each of which is hereby incorporated by reference in its entirety. This type of system uses a microscope-type optical system and moves the stage so as to provide inspection images of either small or relatively large workpieces.
General-purpose precision machine vision inspection systems are generally programmable to provide automated video inspection. Such systems typically include GUI features and predefined image analysis “video tools” such that operation and programming can be performed by “non-expert” operators. For example, U.S. Pat. No. 6,542,180, which is incorporated herein by reference in its entirety, teaches a vision system that uses automated video inspection including the use of various video tools.
Multi-lens variable focal length (VFL) optical systems may be utilized in a precision machine vision inspection system, for example as disclosed in U.S. Pat. No. 9,143,674, which is hereby incorporated herein by reference in its entirety. In various applications utilizing such a VFL optical system, the clear aperture of a VFL lens included in the multi-lens optical system is generally sized such that it does not contribute to excessive vignetting or aberrations, in the precision inspection images that are required from the optical system. However, for some VFL lenses (e.g., such as tunable acoustic gradient (TAG) lenses in particular) the clear aperture (e.g., a distortion free central aperture dimension) depends on the VFL modulation frequency of the VFL lens and the modulation amplitude, and the VFL lens operating frequency (or modulation frequency) and modulation amplitude has been determined and/or limited based at least partly on this design consideration. For some VFL lenses (e.g., such as TAG lenses in particular) the constrained operating frequency outlined above concomitantly determines and/or limits other frequency-dependent characteristics of the VFL lens (e.g., a TAG lens), such as the variation in focus range or optical power that it is able to provide throughout a periodic cycle of the VFL modulation frequency. Related discussion and teachings may be found, for example, in commonly assigned copending U.S. patent application Ser. No. 14/795,409, to Gladnick, which is hereby incorporated herein by reference in its entirety.
As is well known to users of precision machine vision inspection systems, a workpiece surface may come to be positioned outside of a focus range of their optical system, even when it includes a VFL lens. “Searching” and/or attempting to autofocus on such a workpiece surface based on mechanical adjustment of the spacing between the workpiece and the optical system may be slow and/or risky due to the potential for crashing a lens into the workpiece during the “searching” motion. There is a need for high speed operations for searching and/or attempting to autofocus on such a workpiece surface over an extended search range, while at the same time eliminating the risk of crashing a lens into the workpiece during a searching motion.